As a display device which is used for OA appliances such as monitors for word processors, notebook personal computers and personal computers, mobile terminals, television sets and the like, a cathode ray tube (CRT) has been mainly used until now. In recent years, a liquid crystal display device is being widely used in place of CRT because it is slim, lightweight and low in consumed electric power. The liquid crystal display device includes a liquid crystal cell and a polarizing plate. The polarizing plate is in general composed of a protective film and a polarizing film and obtained by dyeing a polarizing film composed of a polyvinyl alcohol film with iodine, stretching it and stacking the both surfaces thereof by a protective film. In a transmission type liquid crystal display device, this polarizing plate is installed on the both sides of the liquid crystal cell, and in some cases, one or more optically compensatory sheets are further disposed. Also, in a reflection type liquid crystal display device, in general, a reflector plate, a liquid crystal cell, one or more optically compensatory sheets and a polarizing plate are disposed in this order. The liquid crystal cell generally has a liquid crystal molecule, two substrates for enclosing it and an electrode layer for applying voltage to the liquid crystal molecule. As to the liquid crystal cell, there are proposed display modes which undergo ON-OFF display in conformity with a difference in alignment state of the liquid crystal molecule and are applicable to any of a transmission type, a reflection type and a semi-transmission type, such as TN (twisted nematic) display mode, an IPS (in-plane switching) display mode, an OCB (optically compensatory bend) display mode, a VA (vertically aligned) display mode, an ECB (electrically controlled birefringence) display mode and an STN (super twisted nematic) display mode.
The optically compensatory sheet is employed in various liquid crystal display devices for the purposes of dissolution of image coloration and enlargement of a viewing angle. As the optically compensatory sheet, a stretched birefringent polymer film has hitherto been used. It has been proposed to use an optically compensatory sheet having an optically anisotropic layer formed of a low molecular or high molecular liquid crystalline compound on a transparent support in place of the optically compensatory sheet composed of a stretched birefringent film. Since the liquid crystalline compound includes various alignment forms, by using a liquid crystalline compound, it has become possible to realize optical properties which could not be obtained through related-art stretched birefringent polymer films. Furthermore, such an optically compensatory sheet functions as a protective film of polarizing plate, too.
On the other hand, image display devices such as CRT, plasma display (PDP), electroluminescence display (ELD), field emission display (FED), surface-conduction electron-emitter display (SED) and liquid crystal display device (LCD) are able to enhance the display performance under a bright room circumstance by preventing glare of an image on the display surface to be caused due to reflection of external light.
Also, in recent years, a market of applications for viewing a video with high image quality from a relatively apart position using a display device having a large-sized screen represented by liquid crystal television sets is quickly spreading. In such applications, the display is required to display a clearer image.
As a method for preventing glare of an image onto the display surface of an image display device, there are disclosed (1) a method for making it difficult to recognize a glared image by imparting performance for vignetting an outline of the glared image (antiglare properties) by surface scattering properties; and (2) a method for making it difficult to recognize a glared image by imparting performance for minimizing the quantity of reflected light (antireflection properties) by providing a low refractive index layer on the most superficial layer of the display surface (see Japanese Patent No. 3507719 (corresponding to U.S. Pat. No. 6,343,865 B1) and JP-A-2003-270409 (corresponding to US2005/0063062A1)).
If the antiglare properties are strong, when a material placed in the surroundings of the display a fluorescent lamp, a human being or the like glares onto the display surface, an outline of the image vignettes, whereby a screen image of the display becomes easy for recognition (the glare is reduced). However, there is a possibility that a whitish-brown color feel of the display surface increases, whereby a contrast in a bright room (bright room contrast) is lowered. Also, in order to increase the visibility in a bright room, devising for strengthening light of backlight, for example, a converging member such as a prism sheet is employed.